Tubes Omitted from Baffle Windows

Frequently, tubes are omitted from the baffle window areas. For this configuration, maldistribution of the fluid as it flows across the bank of tubes may occur as a result of the momentum of the fluid as it flows through the baffle window. For this reason baffle cuts less than 20% of the shell diameter should be used only with caution. Maldistribution will normally be minimized if the fluid velocity in the baffle window is equal or less than the fluid velocity in crossflow across the bundle. This frequently requires baffle cuts greater than 20% of the shell diameter. For such cases, the number of tube rows in crossflow will be less than that assumed in the methods above consequently a correction is required. In addition, the pressure drop for the first and last baffle sections assumes tubes in the baffle windows the factor 2.66 in Equation 8.19 should be reduced to 2.0 for the case of 20% baffle cuts. 

For baffles with no tubes in the baffle windows, the pressure drop for the window section can be calculated as below  

The method presented above can be used to predict pressure drop for banks 

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Tubes have been omitted from the baffle window areas. The performance data are as follows ... 

Sometimes tubes are omitted from the tube bundle in order to provide space for an impingement plate. The impingement plate in this case is generally a circular or rectangular plate located a short distance from the row of tubes and secured by various means. A modification of this type is to form the plate and surround the bundle with it as a partial shroud. Omitting tubes from the tube bundle has many advantages, especially when inlet nozzle velocities are very high or when tubes must be omitted from the baffle windows in order to prevent problems associated with tube vibration. 

Generally impingement plates should not be perforated. There may be times when a flow distribution plate also serves as an impingement plate. There is seldom sufficient space to install a plate to serve both as an impingement plate and as a flow distribution plate unless all tubes have been omitted from the baffle windows. 

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